When amplifying power by using an amplifier such as a high power amplifier (hereinafter referred to as “HPA”) or the like, desired input-output characteristics may not be obtained due to nonlinear distortion characteristics of the amplifier.
Particularly when the frequency of a radio signal to be amplified is high, in order to linearize the amplifier by correcting the nonlinear characteristics thereof, a complex IQ baseband signal of a low frequency, before being converted to the radio signal, needs to be subjected to predistortion as described in Patent Literature 1.
A distortion compensation processing section generates a predistorted signal for canceling the nonlinear distortion of the amplifier by using digital signal processing.
The signal outputted from the distortion compensation processing section is quadrature-modulated by a quadrature modulator, and thereafter, amplified by an HPA.
If a modulation error occurs in the quadrature modulator, a distortion due to the modulation error occurs in the signal for canceling the nonlinear distortion of the HPA. Therefore, the signal for canceling the nonlinear distortion of the HPA does not arrive at the HPA as expected, and the compensation characteristics for the nonlinear distortion of the HPA are deteriorated.
Patent Literature 2 discloses an example of a technique for compensating such a quadrature modulation error.